1. Field of the Invention
The present invention relates generally to separable blade agitators and particularly to glass coated agitators for corrosive service and more particularly glass coated agitators which can be assembled and disassembled within confined areas and introduced and withdrawn from such confined areas through size limiting means of ingress and egress.
2. Background of the Prior Art
Separable blade agitators are well known in the art and are described, for example, in U.S. Pat. Nos. 2,811,339 and 3,494,708. Briefly, a separable blade agitator includes a drive shaft and a separable impeller. The impeller includes a hub for attachment to the drive shaft and two or more blades extending outwardly from the hub. The entire exterior surface of the assembly of the drive shaft, hub and blades exposed to the vessel contents is glass coated to resist corrosion, adherence and abrasion. Such separable blade agitators are used in vessels for mixing various corrosive, adhesive, abrasive or easily contaminated ingredients, such as acids, polymers, pharmaceuticals, dyes and the like.
These agitators especially have great value in closed vessels of this type because they eliminate the need for large vessel openings normally used for agitator removal and admittance. In this respect, the separable impeller portion can be passed into a pressure vessel through a relatively small manhole opening and assembled, within the vessel, to the drive shaft.
Separable blade agitators are useful in either closed or open vessels in that they permit replacement of damaged impellers or changing the size or type of impeller without the need to remove the entire agitator (i.e. drive shaft and impeller) from the vessel or to disconnect the drive shaft from the drive motor and seals.
The primary drawback of separable blade agitators of the above cited prior art is that relatively complicated, expensive components are needed to hold the shaft and agitator portion together in a fluid tight, gasketless connection which is able to transmit torque through the connection from the drive shaft to the impeller blades.
While separable blade agitators having gasketed connections are known, any gasket used must be made from tantalum or other exotic metal or from a fluorocarbon in order to resist the highly corrosive environments to which these gaskets may be exposed. Another drawback with gaskets is that they take a set due to repeated heating and cooling and eventually leak. Normally leaking gaskets can be sealed by retightening but this is difficult to do in separable blade agitators and often the leakage is not noticed until that leakage has reached the interior portions of the assembly, which are not glass coated, and has caused damage to the metal integrity.
Advances have been made in the field of separable blade agitators, to wit, as shown in U.S. Pat. No. 4,221,488 which completely eliminates the need for any gaskets or other assembly components and instead provides a separable blade agitator wherein the impeller is joined to the shaft in a glass-to-glass gasketless connection that is strong enough to transmit torque from the shaft to the agitator blade without the use of any key or spline connection. Thus, even if leakage does occur and corrosive liquids penetrate into the joint, no damage results because all surfaces coming in contact with the corrosive liquid are glass or enamel coated.
The separable blade agitator of U.S. Pat. No. 4,221,488 includes a hollow shaft which is closed at one end, the exterior surface of the shaft being provided with a corrosion resistant coating, such as glass or enamel. The separable impeller includes a hub with a bore therethrough and blades extending outwardly from the hub, the entire surface of the separable impeller, including the internal surface of the hub bore, being provided with a corrosion resistant glass or enamel coating.
The inside diameter of the bore and the outside diameter of the shaft, adapted to receive the impeller, are each machined to provide an interference fit of between 0.00025 and 0.00075 inches per inch of diameter. The assembly of the interference fit is accomplished preferably by super cooling the end of the shaft to shrink its diameter so that it may be inserted into the hub bore. Where three or more blades are desired, more than one impeller can be provided, each having a hub with one or more blades on that hub.
It had been assumed by those skilled in the art that, although there was a tolerance range for machining, grinding and honing the bores of the hubs, for interference fitting them onto the shafts, that where multiple hubs were fitted onto a single shaft, extreme care was required in the final honing to insure that both hubs had precisely the same bore sizes. This was viewed as especially important where the hubs were to be located in immediate proximity to each other. This was assumed because, in interference fitting, the upper limit of the outside diameter of the shaft would be dictated by the smallest diameter hub bore which was fitted over it. Thus, if an additional hub bore, added to the shaft, was larger in diameter, it would merely be relatively loose and would not be capable of bearing the same torque load as the hub that had the smaller diameter bore.
A possible solution to this problem was postulated applicable to a situation where a hollow or tube shaft was used. That postulation was to render the tube wall thin enough to permit sufficient flexibility of the wall so as to permit "necking", i.e., a slightly enlarged diameter of the tube outside diameter which would naturally occur at points on the tube not under the same amount of compressive force as that which was rendered to the tube shaft by the hub with the smallest bore diameter, due to the interference fit. However, this postulation and possible solution was abandoned in view of the fact that the tube is coated with glass, which is well known to be quite brittle. The "necking" was seen to be detrimental because it would tend to produce cracks in the glass, thus allowing corrosion, etc. to reach the underlying metal.
However, commercial considerations were such that the cost of producing sets of hubs with precisely matched bore sizes rendered the concept of multiple hubbed agitators with glassed surfaces, economically unfeasible. Thus, there was a need to be able to develop a system wherein commercial tolerance ranges could be applied to the honing of hub bores, yet still permitting such multiple hubs to be interference fitted to a shaft, both with glass coatings, such that the glass coatings did not crack and the interference fits on each of the hubs was adequate to permit the required torque loading. The market for replacement parts demands that a user be able to replace only one hub and blade assembly, when only one is needed, rather than having to purchase a matched set.